Technique depicts magnetic vortex migration at critical current

MURRAY HILL, N.J.  Bell Labs has demystified how superconductors abandon their resistanceless conduction when they reach their "critical current." Tiny, previously stationary magnetic vortices, containing a single quantum of magnetic flux, begin an organized coherent migration at critical current, which Bell Labs researchers have been able to view for the first time. The new imaging technique is beginning to catch on with other superconductor researchers, and may lead to a better understanding of the high-temperature variety.

A typical superconductor's magnetic vortices are pinnedin a regular array in this Cornell University micrograph

"We are the world's first group to produce pictures of exactly what happens to the tiny magnetic vortices in superconductors at their critical current. Below the critical current the vortices form into a seemingly random stationary pattern, but at critical current they move together as a crystal to form a regular hexagonal array," said Peter Gammel, a distinguished member of the technical staff at Bell Labs. Other senior researchers responsible for the discovery included Dave Bishop, Ernst Bucher and Flavio Pardo of Bell Labs plus Francisco de la Cruz of the Atomic Center, Bariloche, Argentina.

The critical current is the most widely measured phenomenon related to superconductors, because it defines the maximum current load at which there is no resistance in the superconductor. Above the critical current, a superconductor begins to take on resistance values like a regular conductor. The precise mechanism, and the peculiar observable characteristics that cause the onset of critical current could extend the uses and speed the integration of superconducting materials into practical systems.

"In order to make a material superconduct without resistance, you must put something in the material to force the vortices to remain stationary in a random pattern. But when you go above the critical current, it has in some sense broken free of these forces and returns to a more natural state where it has resistance," said Gammel. "The poorly understood manner in which magnetic vortices form in a superconductor has been the main obstacle to widespread use of superconductors in integrated circuits and for industrial uses, such as the science-fictional predictions of levitating trains. Since 1986 researchers have known how to make high-temperature superconducting wires and circuits, but the obstacle presented by magnetic vortices stopped widespread use of the materials.

In detail, when a superconducting material begins to carry current without resistance, that flow induces a magnetic field the same as in normal conductors. In superconductors, however, the magnetic field breaks down into its smallest possible components, each a single quantum of magnetic flux that appears as a microscopic whirlpool or tube. At first these magnetic vortices remain stationary, in a random pattern seemingly determined by the impurities doped in the material. But when the critical current is reached, Bell Labs has now discovered that regardless of the material or doping levels, the vortices move into a regular hexagonal structure that apparently impedes further resistanceless current flow.

"There are differences in the details of how it does it, but in general when you go above the critical current the vortices reform into a beautiful crystalline moving state that impedes the flow of current  so it's essential to future uses of superconductors that we understand how these vortices arrange themselves under varying temperature and magnetic-field conditions," said Gammel.

In nature, the change from disordered arrays to ordered arrays is a common phenomenon, he said. For instance, when geese flock on the ground they form disordered arrays, but when they fly they form into an ordered "V" and stick to that formation as long as they are flying. Likewise, when snow is free-falling, its flakes form disordered arrays, but as soon as the wind blows the flakes reform into uniform arrays, known as "ripples" by atmospheric scientists.

"It's not so much that we discovered this phenomenon  people guessed that something like this was happening, but we were the first observers to devise a method by which we could make pictures of just where these vortices were and what kind of patterns they formed below and above the critical current," said Gammel.

The Bell Labs researchers accomplished their goal using a common method for imaging large-scale magnetic fields. If iron filings are dusted onto a piece of paper, they will align themselves to the field of a nearby magnet to reveal its lines of force. Similarly, in the imaging experiment, the Bell Labs group repeatedly "decorated" the superconductor with microscopic iron particles below the critical current, at the critical current, and above the critical current, to monitor the change in magnetic vortex patterns at the transition point.

"Many people had thought of this method of imaging pictures of magnetic vortices, but everybody agreed that it just wouldn't work. Different researchers all had different reasons for why it wouldn't work, so we just decided to go ahead and try it anyway. The worst thing that could have happened is that we would discover the right reason why it wouldn't work, but instead we were pleasantly surprised with success," said Gammel.

It wasn't an open and shut case, however. The self-induced magnetic vortices did not move even microscopic iron filings, forcing Bell Labs to apply an external magnetic field that was as much as 100 times stronger than the Earth's magnetic field. But once this obstacle was surmounted, the Bell Labs researchers were able to accurately detect the vortices and track their locations in real-time. To create pictures that could be perceived by the human eye, a raft of image-processing algorithms had to be run off-line to add false color to the images obtained.

"Other labs, such as Hitachi's labs in Japan, have used different technologies to make pictures of individual flowing vortices that were further restricted to extremely thin materials, but ours are the first pictures of flowing arrays of magnetic vortices in superconductors." he said. One of the nice aspects of using iron particles is that it places very few constraints on the type of material. "Some people have taken our method and applied it to real technological materials. To use it on a superconducting wire, for instance, all you have to do is polish it down to make it work. It's a very general technique that is widely applicable," said Gammel.

In the spirit of scientific discovery, Lucent Technologies gave the team permission to disseminate the details of the imaging method to other labs. As many as 10 other labs worldwide have taken advantage of the offer and are now using the technique to study the phenomenon of magnetic vortices' migration in superconducting materials.

"We have been very generous, by providing details about how our imaging method works. We even supply drawings of our equipment as to how to set up your lab to duplicate our methods. I consider it a great tribute that when I visit other labs now, they have duplicates of our setup. Superconductivity is such an important phenomenon, and so few groups are working on it, that the more help we give each other to explore it, the better off everybody will be," said Gammel.